Food manufacturers are constantly looking for a new improved method of preparing foods. In particular, a rapid and efficient method which also produces high quality foods is always desirable. Special problems, however, are encountered by manufacturers when hot oil heating (deep frying) is involved in cooking and/or heating food products such as egg rolls, pizza rolls, fried taco rolls, fried burritos, corn dogs or break fast rolls etc. which generally consists of a wrapper (outer shell/skin) and an inner filling. Such food items are difficult to cook and/or heat rapidly while at the same time, yielding desirable crispiness of the wrapper (outer shell/skin) and a high quality cooking and/or heating of the inner filler. In general, if a relatively high temperature is applied in the conventional deep-fry cooking and/or heating, the wrapper (outer shell/skin) becomes overcooked and burnt before the inner filling even reaches the desired temperature. This is particularly true when the products are frozen prior to processing.
When heating the somewhat cylindrical food products mentioned above by means of the conventional oil heating alone, it is known from experience in the applicant's egg roll manufacturing plant that for a product of one and one-half inches in diameter, the time required to reach the core temperature of 160 degrees fahrenheit(F.) from an initial temperature of 50 degrees F. is about five minutes with the oil temperature being maintained at 355 degrees F. If the same product is frozen prior to processing, it becomes impossible to obtain a satisfactory result by frying alone. For a thin and uniform food item such as french fries, it does not constitute a problem even if they are frozen prior to deep frying. But for the same frozen egg roll mentioned above with an initial temperature of 15 degrees F. and the same frying temperature, the time required to reach a core temperature of 160 degrees F. is about 10 minutes. This core temperature level is required by the U.S. Department of Agriculture meat and poultry inspection on items such as chicken egg rolls. At this frying time, however, the outer wrapper of the egg roll becomes burnt long before the core temperature reaches 160 degrees F. The present invention is a solution to the problem of cooking and/or heating a frozen or non-frozen food product in a relatively short time is to combine conventional oil heating and microwave heating simultaneously during the entire or partial period of the said cooking and/or heating time.
The microwave is widely used to cook and/or heat food products such as frozen dinners or even frozen egg rolls mainly for its relatively shorter heating time and for simplicity of operation. In the case of heating an egg roll for example, the use of microwave heating alone yields undesirable result. The microwave excites water molecules of the inner filling, thus generating heat and water vapor which causes the egg roll wrapper to become soggy. In general, a food product having a wrapper (outer shell/skin) and an inner filling with moisture cannot be cooked and/or heated satisfactorily with microwaves alone, if it is desired to have a crispy outer crust as the result of cooking and/or heating. This is also true, even for a food item with uniform inner and outer substances such as a french fry where a similar requirement, namely, "a crispy outer layer" is sought after cooking and/or heating.
The above argument supports the necessity of the present invention for using both conventional oil heating and microwave heating for a relatively rapid and better quality cooking and/or heating of a food product consisting of a wrapper (outer shell/skin) and an inner filling. It is apparent that a simultaneous application of the two heating methods will not only further reduce cooking and/or heating time, but also ensure the crispiness of the outer crust after said cooking and/or heating. It should be noted that in the case of cooking egg rolls (or other similar food products), it is not desirable to use microwave cooking prior to a conventional oil frying. An egg roll heated by microwaves yields a soggy egg roll wrapper, a soggy egg roll wrapper breaks easily during deep frying. The only workable method, in this case is to apply the two heating methods simultaneously.